The invention concerns a composite membrane for the separation of water vapors from bases, comprising a microporous support membrane and a separating layer disposed thereon.
Water vapors are presently separated from process gas mixtures utilizing absorption processes (for example, glycol dryers) by an adsorption process (for example a molecular sieve process, adsorption to activated aluminum oxide or silica gel) or by refrigeration dryers. Because of the low flow densities and the high product losses, the last mentioned devices are used only with low or medium volume flows.
Absorption processes such as gas drying by means of glycol washers (TEG, DEG) are used in large volume manufacturing processes for example for the processing of natural gas. After being charged with water, the absorption material is regenerated in a second process step. This known process operates at high separation rates and is used therefore particularly for the drying of natural gas.
However, this known absorption process requires a relatively complex processing procedure with a need for a relatively large number of operating personnel and also high investment costs. Furthermore, corrosion and fouling, particularly in the needed heat exchangers result in operational problems. Also, additional costs are incurred by glycol losses and the necessary additives such as antifoaming chemicals and chemicals for the adjustment of the pH value.
With the known adsorption drying process, adsorbing means such as molecular sieves or silica gel are first charged with moisture and, in a second step, the moisture is again removed. In the first step, moisture is collected by the adsorption means and, in a second step, the adsorption means are regenerated by heat, by pressure change or by a dry gas stream. This known process is particularly suitable for the drying of streams with a low moisture content. Also, this known method permits the removal of moisture to a high degree resulting in a low remaining moisture content in the product.
It is a disadvantage of the known methods that the adsorption means need to be regenerated cyclically so that the moisture removal process is discontinuous. For the separation of high moisture loads, this process is therefore too expensive.
Refrigeration dryers are often used for the continuous separation of water vapors from gas mixtures. The dew points that can be obtained thereby are about 2.degree. C. since ice is formed on the cooling coils at lower temperatures. However, lower dew points can be obtained by the use of two parallel apparatus of which one is de-iced while the other is operated to remove moisture. This arrangement however requires high investment costs and has relatively high energy consumption. Consequently, the limits of refrigeration dryers are near the freezing point.
The membrane processes known in the art could, in principle, also be used for the dehumidification of large amounts of gas. However, with the membrane separation, there are always two outlet flows, that is, the cleaned product flow and the moisture loaded permeate flow. Depending on the application, the permeate flow can be returned to the process or it can be rejected. In any case, a high permeate flow results in relatively high operating expenses.
It is the object of the present invention to provide a new membrane which permits high separation rates, which has a good mechanical and chemical stability and which can be used for the separation of water vapors from gas mixtures.